Conventional roof coverings for sloped roofs include asphalt shingles, wooden shake shingles, sheet metal, slate, clay and concrete tile. Clay and slate are advantageous because of their high weatherability and desirable appearance. Various parts of the world have local or regional architectural preferences for the appearance of a roof. In Europe, clay tile and slate are generally preferred over the relatively flat looking asphalt shingle. Tastes in the U.S. vary, with the western and southwestern part of the U.S. preferring clay tile or wooden shakes and shake shingles.
One of the problems with clay tile and slate roofs is that the clay and slate tiles must be carefully fastened individually to the roof, and therefore they are expensive to install. Also, clay and slate tiles are inherently fragile, and suffer much breakage during manufacturing, shipping and installation. These materials are fragile even after installation on the roof, and can be damaged by foot traffic on the roof. One of the problems with wood shake shingles is that they are not naturally fire resistant, and therefore are prohibited in certain fire-prone areas, or at least require treatment for fire resistance.
Molded plastic shingles have been developed as an alternative both to clay and slate tiles, and to wooden shake tiles. U.S. Pat. No. 5,615,523 to Wells et al., which is herein incorporated by reference, discloses such shingles. Typical compositions include about 20 percent resinous material and about 80 percent fine stone filling and glass fiber reinforcement, with minor amounts of additives. This class of materials is also known as polymer concrete or highly filled resin. The high percentage of filler is important for maintaining a high fire resistance. Typically, these highly filled plastic shingles are compression molded, although casting, injection molding, injection compression molding, and transfer molding can also be used.
In general, the wear surfaces of compression molds are relatively durable where the moldable material is a typical fiber reinforced plastic (FRP) material. However, when the moldable material is highly filled with an inherently abrasive filler material, as is the case with the highly filled plastic material used to mold roofing shingles and tiles, the mold surfaces can wear out at a relatively rapid rate. Erosion of the mold cavity surface necessitates removal of the mold for refurbishing. This is a time consuming and expensive process.
It would be advantageous if there could be developed a molding process that uses relatively inexpensive facings or inserts so that when the molding surface wears out the surface can be replaced by a fresh insert without replacing the whole mold. Further, such a molding process would improve upon the conventional milling and machining methods used in imparting the desired surface features into the surface of mold. Also, it would be beneficial if a method for producing the mold inserts could be developed. Such a method would eliminate the need for an expensive milling process to prepare the molding surface in the inserts.